I. Field of the Invention
The present invention relates to cellular communication systems. More specifically, the present invention relates to a method and apparatus providing a communication link quality indication within a communication system, and in a particular case within a code division multiple access (CDMA) cellular communication system, thereby enabling improved signal transmission quality within the system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA) frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307 entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, the disclosure thereof incorporated by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users each having a transceiver communicate through satellite repeaters or terrestrial base stations (also known as cell-sites stations, or for short cell-sites) using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access techniques. In a CDMA system, increases in system capacity may be realized by controlling the transmitter power of the portable units associated with each user so as to reduce interference to other system users.
The CDMA receivers of the cell-site operate by converting a wideband CDMA signal from a corresponding one of the portable unit transmitters into a narrow band digital information carrying signal. At the same time, other received CDMA signals using the same frequency that are not selected remain as wideband noise signals. The bit-error-rate performance of the cell-site receiver is thus determined by the ratio of the power of the desired signal to that of the undesired signals received at the cell-site, i.e., the received signal power in the desired signal transmitted by the selected portable unit transmitter to that of the received signal power in undesired signals transmitted by the other portable unit transmitters. The bandwidth reduction processing, a correlation process which results in what is commonly called "processing gain", increases the signal to noise interference ratio from a negative value to a positive value thus allowing operation within an acceptable bit-error-rate.
In a terrestrial CDMA cellular communication system it is extremely desirable to maximize the capacity in terms of the number of simultaneous communication links capable of being supported by a given system bandwidth. System capacity can be maximized if the transmitter power of each portable unit is controlled such that the transmitted signal arrives at the cell-site receiver at the minimal signal to noise interference ratio which allows acceptable data recovery. If a signal transmitted by a portable unit arrives at the cell-site receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications. On the other hand if the mobile unit transmitted signal is at a power level that is too high when received at the cell-site receiver, communication with this particular mobile unit will be acceptable. However, this high power signal acts as interference to other mobile unit transmitted signals that are sharing the same channel, i.e. frequency spectrum. This interference may adversely affect communications with other portable units unless the total number of communicating portable units is reduced.
In the terrestrial application of CDMA communication techniques, the portable unit (e.g., mobile telephone or personal communication instrument) transceiver measures the power level of the signal received from a cell-site station. Using this power measurement, the portable unit transceiver can estimate the path loss of the channel between the portable unit and the cell-site station. The portable unit transceiver then determines the appropriate transmitter power to be used for signal transmissions between the portable unit and the cell-site station, taking into account the path loss measurement, the transmitted data rate and the cell-site receiver sensitivity.
The signals received from each portable unit at the cell-site station are measured, and the measurement results compared with a desired power level. Based on this comparison the cell-site determines the deviation in the received power level from that which is necessary to maintain the desired communications. Preferably the desired power level is a minimum power level necessary to maintain quality communications so as to result in a reduction in system interference. Instead of measuring the signal strength of each signal comparing the result with a desired power level, other criteria could be used to determine the power adjustment commands. For instance the criteria could be a signal-to-noise ratio, data error rate, or audio quality.
The cell-site station then transmits a power control command signal to each system user so as to adjust or "fine tune" the transmit power of the portable unit. This command signal is used by the portable unit to change the transmit power level closer to a predefined level required to sustain communication on the reverse link i.e. the link from the portable unit to the cell-site. As channel conditions change, typically due to motion of the portable unit, both the portable unit receiver power measurement and the power control feedback from the cell-site station continually readjust the transmit power level so as to maintain a proper power level.
In a terrestrial CDMA communication system the maximum range at which communication may be supported between the cell-site and a particular portable unit is proportional to the power capable of being transmitted by the portable unit on the reverse link. Although existing techniques for power control provide for acceptable communication quality when the portable unit is displaced from the cell-site by less than its maximum range of transmission, the maximum transmission range on the reverse link could be increased if a user were provided an indication of which orientations of the portable unit resulted in higher transmission gain on the reverse link.
Unfortunately, however, known CDMA power control techniques do not provide means for adjusting the position or orientation of the portable unit so as to increase the strength of the signal transmitted over the reverse link from a portable unit to the cell-site. A primary reason for providing such control would be to improve signal transmission in those instances where the portable unit is separated from the cell-site by a distance approximately equivalent to the maximum transmission range of the portable unit. If in such locations the reverse channel link is disadvantaged the maximum transmission power of the portable unit may be insufficient to provide a reverse link signal of the requisite strength to the cell-site. Accordingly, the cell-site would operate to send a continuous stream of power control command signals to the portable unit specifying that the power of the signal transmitted thereby be increased. This would continue until, for example, transmission conditions on the reverse link improved or the orientation of the portable unit were adjusted so as to increase the strength of the signal received by the cell-site. Under such circumstances there is an increased likelihood of an abrupt "break", i.e., extinction, of the communication link between the cell-site and the portable unit, the occurrence of which obviously degrades system performance.
It is therefore an object of the present invention to provide a novel and improved method and apparatus for improving signal transmission quality in a CDMA communication system by supplying a user with an indication of communication quality over the reverse link, thereby enabling the orientation of the portable unit to be adjusted by the user so as to maximize reverse link transmission gain.